A photolithography process is conducted by using various types of liquid treatment units such as a coating unit for performing a film formation to a substrate by supplying a coating liquid to a surface of a substrate, a developing unit for performing a development treatment by supplying a developing solution to a surface of a substrate, and a cleaning unit for cleaning the front and back surfaces of a substrate before loading the substrate into an exposure apparatus. The liquid treatment processes performed by such treatment units include the back surface cleaning. The back surface cleaning is performed by discharging a cleaning liquid or pure water onto the substrate from a cleaning nozzle fixedly mounted to the treatment unit, while rotating the substrate with the substrate being held on a chuck by vacuum suctioning.
An example of the back surface cleaning will be explained below in connection with a cleaning unit shown in FIG. 10. The cleaning unit includes a cup 100 in which a wafer W is contained in a horizontal posture, a chuck 102 located at the center of the cup and driven for rotation by a motor M, cleaning nozzles 104a and 104b for discharging the cleaning liquid toward the back surface of the wafer W, and a supply nozzle 105 for supplying the cleaning liquid to the front surface of the wafer W. The back surface cleaning of the wafer W is carried out by supplying the cleaning liquid onto the back surface of the wafer W, when a treatment of the wafer W is being performed by supplying a treatment liquid to the front surface of the wafer W, or after completion the treatment. An example of the wafer back surface cleaning performed by using a similar configuration is described in JPH10-172944A (see FIG. 7 thereof, for example).
After such back surface cleaning, a considerable amount of cleaning liquid remains extensively adhering to the back surface of the substrate as a result of the discharging of the cleaning liquid onto the rotating substrate. Since the cleaning liquid adhering to the back surface of the substrate may cause adhesion of particles to the substrate and contamination of a transfer arm, the substrate should be dried before unloading the substrate from the cleaning unit. The drying of the substrate is performed by, for example, rotating the cleaned substrate at a high speed (e.g., 1500 rpm) and making the adhering cleaning liquid scatter around due to the centrifugal force (hereinafter referred to as “spin drying”.
Problems with the conventional technology will be explained below by referring to FIGS. 1 and 2. With the progress of the miniaturization of the circuit patterns in recent years, the patterns are often formed up to a region close to the periphery of the wafer W (e.g., 1 mm inside the periphery) in order to obtain a greater number of devices from one substrate. FIG. 1 shows an example in which a coating film T has been formed on the surface of the wafer W, and a peripheral film removal in 1 mm width has been performed to the peripheral part S of the wafer W. The wafer W must have a notch part N. As shown in FIG. 2 magnifying a part X around the notch part N, the notch part N is a cutout having a length of 1 mm to 1.25 mm toward the inside of the wafer W and a width of 1 mm in the circumferential direction at the edge of the wafer W (upper edge of a bevel part B).
FIG. 2 shows the state of a region around the notch part N after performing the spin drying of the wafer W after the back surface cleaning. Part of the film within 1 mm from the edge of the wafer W has been removed by a previous liquid treatment. The left-hand side of the notch part N is in fine condition. The cross-hatched region C above the dotted line A on the right-hand side of the notch part N is defective. The cross-hatched region C is located in the device region of the wafer W and a defect existing in the device region can adversely affect the device patterns formed on the wafer W.
The cross-hatched region C corresponds to traces (water marks) of the cleaning liquid (adhering to the back surface of the wafer W in the wafer back surface cleaning) penetrating into the front surface of the wafer W via the notch part N in the spin drying. Such penetration of the cleaning liquid into the front surface occurs due to change in the behavior of the cleaning liquid caused by the notch part N when the cleaning liquid adhering to the back surface of the wafer W is thrown off by the centrifugal force. Specifically, the cleaning liquid is captured within the thickness of the wafer W in the V-shaped valley part of the notch N. The captured cleaning liquid swells up due to surface tension and part of the swollen cleaning liquid breaks away and penetrates into the front surface of the wafer W. The cleaning liquid penetrating into the front surface of the wafer W and forming a ball is immediately thrown off by the centrifugal force. Since the wafer W is rotating, such a ball-shaped cleaning liquid develops exclusively in a region ahead of the notch part N in regard to the wafer rotation direction, the defective cross-hatched region C develops in a style like the one shown in FIG. 2.
In order to suppress such defects specific to the cleaning liquid drying step, a drying step recipe for changing the revolution speed of the spin drying with time is set. For example, the majority of the cleaning liquid adhering to the back surface is removed first by rotating the wafer W at a low revolution speed around 400 rpm. Subsequently, the revolution speed is successively shifted to a middle revolution speed around 1000 rpm and a high revolution speed around 1500 rpm and the drying is gradually advanced in multiple stages. With this drying step recipe, the gathering and swelling of the cleaning liquid in the notch part N can be suppressed. However, setting the drying step recipe in multiple stages leads to extension of the processing time and that adversely affects the productivity.